This invention relates to the positioning of objects and, more particularly, to a system which uses shape memory metal alloys to accurately point or align a movable piece of equipment or other object.
Many applications, especially those in the field of aerospace, require an actuator that is capable of positioning an object with a high degree of accuracy. Past efforts to provide such a precision actuator have relied upon various mechanical, hydraulic and electrical devices, such as screws, gears, hydraulic pumps, and direct drive or geared motors. It has been difficult, however, to provide exact positioning of an object with these approaches because of the mechanical errors introduced by the hydraulics, motor drives, gears, and other mechanical components. As a consequence, these prior systems have not been entirely satisfactory.
A series of shape memory metal alloys possess a unique mechanical "memory" which, given the proper conditions, can be restored to their original shape after being "permanently" deformed out of that shape. Notable among these materials is Nitinol, which is an alloy of nickel and titanium. Generally, these alloys have chemical compositions and range from about 53 to 57 weight percent nickel balanced with titanium.
The memory of these materials, i.e., the return of the materials to their original shape, is triggered by heating the alloy to a moderate temperature known as the austenite start temperature, A.sub.s. When heated to this temperature, the alloy begins a transformation from a soft, martensitic state to a harder, more dense austenitic state. This transformation continues in a linear manner as further heat is applied up to an ending temperature, known as the austenite final temperature A.sub.f, at which the alloy is wholly transformed to austenite. As a consequence of this transformation characteristic, martensitic Nitinol can be plastically deformed from an original shape into an intermediate shape, and then caused to return to the original state by heating it to its transition temperature.
The particular austenite start temperature at which the material returns to its original shape is governed by the nickel/titanium ratio. For Nitinol, this transition temperature can be varied from about -200.degree. C. to over +120.degree. C. Even higher transition temperatures can be provided by doping the Nitinol with other metals. For example, pladimum-doped alloys have transition temperatures in excess of 230.degree. C.
Considerable force is exerted by a memory metal alloy as it returns to its original, preset shape. The force so generated is capable of doing significant mechanical work. The amount of work performed and the associated recovery stresses depend on the amount of strain that is induced when the material is plastically deformed from its preset memory shape to the intermediate shape. A Nitinol alloy shape can be strained up to 8% and recovered to the memory set shape upon the application of heat sufficient to meet the alloy's set memory point temperature.
Nitinol has several other advantageous mechanical and physical properties. An interesting feature of the shape memory function of Nitinol is that the speed of recovery to the set shape can be controlled by the rate at which thermal energy is applied. This control capability can be used to stop a shape at any intermediate point during the recovery operation. Thus, with precise thermal control, the speed with which the material moves and the amount of movement of the material can be regulated in a desired manner. Further useful properties of Nitinol include excellent damping characteristics and an electrical resistivity that changes in a predictable fashion over the transformation-temperature range.